Body size has a pervasive effect on animal functioning and life history with size dependent changes in performance and physiology throughout ontogeny being common in many ectothermic vertebrates. However, as selection on juvenile life history stages is strong, juveniles often offset the disadvantages of small body size by disproportionate levels of performance. Here, we investigate size-related changes in defensive strike performance in an arboreal pit viper, Trimerusurus (Cryptelytrops) albolabris. Our data show a significant negative allometry in the scaling of head dimensions and head mass to body mass. However, strike velocity and strike distance are independent of body mass, with juveniles in our sample striking as fast and as far as adults. In contrast to model predictions suggesting that acceleration capacity should decrease with increasing body mass, acceleration capacity increases with snake body mass. Our results suggest that this is the result of a negative allometric scaling of head mass combined with an isometric scaling of the dorsal epaxial musculature. Finally, our data show a significant sexual dimorphism in body size and strike velocity with females being heavier and striking faster independent of the dimorphism in body size.